South Park: The Stick of Truth - Flower for a Princess

You can get this sidequest when you first visit Kupa Keep, which is Cartman's lair in his backyard, during The New Kid in Town main story quest at the beginning of the game. Talk to Princess Kenny to receive the quest and you can find the flower on the left side of the area. Bring it back to Kenny to finish the quest, who will reward you by becoming your friend on Facebook.